OK, so 'no maximum' doesn't entirely mean 'no maximum'. It means SMF doesn't check the length, and it's possible that you're running into some of the internal not-obvious limits of the database underneath.
As mentioned if you haven't already converted to UTF-8, you should take a backup and then do that... once that's done, you should also change the maximum to 70000 and then back to 0. This will force the table to get bigger under the hood so it won't cut off (or error out) if you try to put too much content in there.